Silica matting agents

ABSTRACT

A silica matting agent comprises particulate amorphous silica in which the particles of silica have been treated with a hydrophilic polyolefin wax.

This invention relates to silica matting agents and, in particular towax-coated silica matting agents.

Wax-coated silica matting agents have been used in paints, varnishes andlacquers. Waxes which have been used in preparing such matting agentsinclude microcrystalline waxes and synthetic polyethylene waxes. Whenused in clear films such as solvent-based lacquers for wood, the knownmatting agents give good film smoothness and transparency, but it isdesirable to improve upon the performance of existing matting agents.

It has now been surprisingly found that the use of certain polyolefinwaxes to coat silica provides matting agents having a superiorperformance, particularly in clear solvent-based lacquers designed to beused on wood.

According to the invention, a silica matting agent comprises particulateamorphous silica characterised in that the particles of silica have beentreated with a hydrophilic polyolefin wax.

The silica of the invention is treated with a hydrophilic polyolefin waxand is generally referred to as a wax-coated silica. This does notnecessarily imply that all the silica particles are coated with wax orthat the particles are uniformly or completely coated with wax.

It has been found that, in comparison to conventional wax-coatedsilicas, the products of the invention possess better wetting propertieswhen mixed with a resin system and produce a cured resin film withbetter transparency and smoothness, as demonstrated by a hand-feel test.

The polyolefin wax can be a polymer or copolymer of any olefin, such asethylene or propylene, but preferred polyolefin waxes are polyethylenewaxes.

Particularly preferred hydrophilic polyethylene waxes for use intreating silica to prepare the matting agents of this invention includethose sold by Cray Valley Speke as Crodawax WN 1442 and by LubrizolCoating Additives GmbH as Lanco™ PEW 1555.

The preparation of polar and, therefore, hydrophilic polyolefin waxes,is well known. For example, a non-polar polyethylene wax can be oxidisedby passing air into molten wax or into wax in an inert dispersionmedium. Alternatively, a polyethylene plastic can be degradedoxidatively under conditions which result in both chain cleavage andoxidation. Oxidative degradation can be carried out by passing air oroxygen-enriched air Into a dispersion of polyethylene plastic or oversolid plastic at elevated temperature. The high pressurecopolymerisation of ethylene with oxygen-containing comonomers,especially acrylic acid, produces a polar form of polyethylene and themolecular weight can be controlled to produce a wax. Polar polyolefinscan also be produced by radical grafting of polar unsaturated monomersonto non-polar polyolefin waxes. By these means, it is believed thatoxygen-containing functional groups, such as carboxyl, carbonyl, esterand hydroxy groups are introduced into the wax molecule.

In addition to synthetic modification of a polyolefin wax as outlinedabove, the hydrophilicity of both unmodified and modified polyolefinwaxes can be increased by the blending of the wax with compatiblehydrophilic materials, such as surfactants or non-olefinic hydrophilicpolymers. The blending of such materials is generally intimate; forexample the materials are mixed when molten and cooled to give a solid,continuous, homogeneous material which can be easily processed furtherby physical means.

Polar polyolefin waxes usually have an add number in the range 10 to 110mg KOH per g and, frequently, in the range 10 to 60 mg KOH per g, whichreflects the presence of, for example, carboxyl groups in the polymer.However, where the hydrophilicity of the polyolefin wax is a result ofblending hydrophilic materials with the wax, the acid number isfrequently lower than for modified waxes and may be in the range 1 to 60mg KOH per g.

The weight average molecular weight of hydrophilic polyolefin waxessuitable for use in this invention is generally in the range 800 to30000 and preferably is In the range 800 to 10000.

Usually the melting point of the hydrophilic wax is in the range 80 to160° C. and preferred waxes have a melting point in the range 100 to130° C.

The amount of hydrophilic polyolefin wax present in the matting agent ofthe invention is preferably at least 3 percent by weight with respect tountreated silica and, usually, the amount is less than 20 percent byweight with respect to untreated silica. More preferably, the amount ofhydrophilic polyolefin wax present is in the range 5 to 12 percent byweight with respect to untreated silica.

Generally, the silica used to produce the matting agent can be anysilica which has found use as a matting agent Silicas prepared by thegel process or the precipitation process can be used.

Usually, the matting agent will have a weight mean particle size, asmeasured by Malvern Mastersizer® in the range 2 to 20 μm. Preferably,the particle size of the matting agent, as measured by MalvernMastersizer®, is in the range 3 to 8 μm. Preferably, 90 percent byweight of the particles have a size below 25 μm and, more preferably, 90percent of the particles by weight are below 12 μm.

The BET surface area of the untreated silica is generally in the range100 to 800 m²g⁻¹ and preferably in the range 200 to 600 m²g⁻¹.

The pore volume to nitrogen of the untreated silica is preferablygreater than 1.0 cm³g⁻¹ and more preferably greater than 1.5 cm³g⁻¹.Usually, the pore volume to nitrogen will not be greater than 2.1cm³g⁻¹.

Usually, the untreated silica will have an oil absorption greater than100 cm³/100 g, and, preferably, the oil absorption will be in the range100 to 500 cm³/100 g.

The silica can be treated with the hydrophilic polyolefin wax using anymethod which provides a product in which the wax is reasonably uniformlymixed with the silica. A preferred method comprises passing the silicaand the wax concurrently through a size reduction apparatus such as amicroniser or a jet mill. Usually, when such processes are used toprepare silica matting agents according to the invention, it ispreferable to use a hydrophilic wax having a small particle size, suchas micronised wax, for the feedstock. A suitable wax has an averageparticle size below 15 μm. Generally, the silica feedstock will have anaverage particle size as measured by Malvern Mastersizer® in the range30 to 300 μm. In a preferred method, the wax and the silica arethoroughly blended in appropriate proportions by mixing in aconventional blender before feeding to the microniser or mill.Alternatively, the wax and silica can be separately fed at appropriaterates to the microniser or mill. The operating conditions of the millare fixed so as to ensure that the mixture of silica and wax reaches atemperature above the melting point of the wax as it passes through themicroniser or mill. Usually, the mixture will be heated to a temperaturein the range 100 to 150° C. during the milling/treatment process. Thesilica is also reduced in size during the micronising or milling processso that the treated silica has a particle size appropriate to its use asa matting agent usually within the limits mentioned hereinbefore.

The matting agents of the invention are suitable for use in a variety ofcoating systems such as paints, varnishes and lacquers, that is,generally in any system in which conventional silica matting agents havebeen used. They are particularly suitable for use in clear solvent basedlacquer systems such as those designed to be used on wood. Typically,such systems include acrylic, polyester, acrylic/polyurethane,polyester/polyurethane, alkyd urethane, alkyd/urea or nitrocellulosesystems. The matting agents of the Invention have been shown to performwell In such systems, as measured by low and soft sedimentation of thelacquer on standing, low gloss-up on polishing, scratch resistance, timeto wet the matting agent, dispersibility and smoothness of the film, asdemonstrated by hand-feel.

Tests used to characterise the matting agents of the invention and theirperformance in coating systems are described below.

Weight Average Particle Size

The weight mean particle size of the silica is determined using aMalvern Mastersizer® model S, with a 300 RF lens and MS17 samplepresentation unit. This instrument, made by Malvern Instruments,Malvern, Worcestershire, uses the principle of Fraunhofer diffraction,utilising a low power He/Ne laser. Before measurement the sample isdispersed ultrasonically in water for 5 minutes to form an aqueoussuspension. The Malvern Mastersizer® measures the weight particle sizedistribution of the silica. The weight mean particle size (d₅₀) or 50percentile is easily obtained from the data generated by the instrument.Other percentiles, such as the 90 percentile (d₉₀), are readilyobtained.

Surface Area and Pore Volume.

Surface area and pore volume of the silica were measured using standardnitrogen adsorption methods of Brunauer, Emmett and Teller (BET) using amulti-point method with an ASAP 2400 apparatus supplied by Micromeriticsof USA. The method is consistent with the paper by S. Brunauer, P. H.Emmett and E. Teller, J. Am. Chem. Soc., 60, 309 (1938). Samples wereoutgassed under vacuum at 270° C. for 1 hour before measurement at about−196° C.

Oil Absorption

The oil absorption is determined by the ASTM spatula rub-out method(American Society of Test Material Standards D 281). The test is basedon the principle of mixing linseed oil with the silica by rubbing with aspatula on a smooth surface until a stiff putty-like paste is formedwhich will not break or separate when it is cut with a spatula. The oilabsorption is then calculated from the volume of oil (V cm³) used toachieve this condition and the weight, W, In grams of silica by means ofthe equation:Oil absorption=(V×100)/W, i.e. expressed in terms of cm³ oil/100 gsilica.pH

This measurement is carried out on a 5 weight percent suspension of thesilica in boiled demineralised water (CO₂ free).

Dispersibility

The dispersibility of the silica matting agent is measured by a“fineness of grind” test. The silica is dispersed in the chosen lacquer(as defined in the examples) at high shear using a Cowles-type stirrerblade in a mixing vessel having parameters as follows; Ratio of CowlesBlade Diameter: Vessel diameter: Vessel Height: Depth of fill: Height ofrotor in vessel (from base): distance from blade Up to vertical vesselwall=1:2.5:2.5:1:0.5:0.5. The blade tip speed is 6.3 m/s. The mixture isstirred for 10 minutes, after which the fineness of grind is measured bydrawing down the dispersion onto a 0-50 μm Hegman Gauge and viewing thedrawn down layer with the aid of a display cabinet. The result(expressed in micrometers) is indicative of the level of dispersion.

Matting Efficiency

The matting efficiency of silica matting agents is determined bydispersing the silica in the chosen solvent based lacquer (as defined inthe examples), using the same conditions as described in theDispersibility test and drawing a 40 μm wet film on a Leneta card or ablack glass plate, drying the film as appropriate to the lacquer systememployed and measuring the gloss using a Byk multigloss meter at theangle specified in the examples.

Wet-In Properties

5 g of the silica matting agent to be tested is added in one fluidmovement to a beaker containing 100 g of the chosen lacquer (asspecified in the examples) while this is being stirred at 500 rpm usinga four-blade propellor with a diameter of 4 cm using the mixingparameters given above in the Dispersibility test. The time taken forthe silica powder to disappear from the lacquer surface is noted. Thestirring is continued at the same rate for a total of 3 minutes. Thenthe fineness of grind is measured by drawing down the resultingdispersion onto a 0-50 μm Hegman Gauge and viewing the drawn down layerwith the aid of a display cabinet.

Sedimentation Rate

Information on settlement behaviour and the nature of the particulatebed formed is obtained by dispersing the matting agents in the paint orlacquer (as described in the examples) at a concentration of 4 percentby weight. The resulting paint or lacquer containing matting agent isthen stored in a graduated cylinder at room temperature. After one weekthe height of the sediment apparent in the paint or lacquer is measuredand recorded as a percentage of total height of the paint or lacquer inthe cylinder. To assess the redispersibility of the sediment formed, themeasuring cylinder is inverted by hand and the number of inversionsrequired to remove any trace of sediment from the base of the cylinderrecorded. An Inversion is defined as a “taking an upright measuringcylinder containing lacquer and sediment, inverting to 180° verticallythen subsequently returning the cylinder to a fully upright position”.On this basis, a “soft” sediment may only require 1 inversion while a“harder” sediment may require many inversions.

Film Optical Properties

The optical properties of a lacquer film are assessed by the Haze,Clarity and Transparency of the film. These are determined by dispersingthe silica in the chosen solvent based lacquer (as described in theexamples), drawing a film at 100 μm wet film thickness on a clear glassplate, drying the film as appropriate for the resin system being usedand measuring the gloss using a Byk Haze-gard PLUS meter (Cat. No.4725). This meter measures the transmittance, clarity and haze of afilm. Transmittance is defined as the ratio of total transmitted lightto incident light (this is reduced by reflectance and absorbance).Clarity is defined as narrow angle scattering. More specifically,clarity is the percentage of transmitted light that deviates from theincident by less than 2.50 on average. Haze is defined as wide anglescattering. More specifically, haze Is the percentage of transmittedlight that deviates from the incident by greater than 2.5°. For clearfilms, such as those used for wood coatings, It is desirable thattransmittance and clarity are maximised while haze is minimised.

Gloss-up on Polishing

A 100 μm film of paint or lacquer containing the matting agent (asdescribed in the examples) is drawn onto an Leneta 7B card as substrateand the 60° gloss value is measured with a gloss meter as describedabove for Matting Efficiency. The lacquered substrate is then mounted ona Taber Abraser Model 5151 fitted with felt wheels (model CS-5). Thelacquer is subjected to a measured number of revolutions of the wheelsand the gloss is measured again in the polished area. The lacquer isabraded over a number of cycles to obtain a “glossing up” curve. A graphof gloss at 60° against number of revolutions is plotted and fittedusing the following logarithmic equation:y=a×[ln(n)]+b,where y=60° gloss, n=number of revolutions and b is a numerical valueobtained by fitting the curve. The parameter “b” is related to mattingefficiency. The parameter “a” is defined as the “rate of gloss-up”. Thefit of this curve is also determined by calculating the square of thePearson product (R²). If R²=1, the profile of the glossing up curve fitsthe above equation exactly and is described as logarithmic (i.e. glossvalues increase rapidly during initial abrasion then plateau to aconstant level). If the value of R² is significantly less than 1, suchas 0.65, then the profile of the glossing up curve can be described aslinear (i.e. gloss values increase in a linear fashion over the wholeperiod of the abrasion cycles). Ideally, for lacquers, the rate ofglossing-up of a matting agent (value of a) should be as low as possibleand the profile of the glossing-up curve should be linear.Hand-Feel

Hand-feel is a subjective test which describes the surface texture asperceived by touch. A dry lacquer is generally considered to have goodhand-feel if the surface can be described as having a “dry velvet touch”when any part of a dry hand is drawn across the surface. Paints orlacquers are prepared as described in the examples and films are drawnonto black glass plates at a wet film thickness of 100 μm using a Birdtype applicator. The hand feel is evaluated subjectively by a panel ofat least 4 experienced people by comparison with appropriate standardplates prepared using commercial matting agents known to give goodhand-feel. A single arbitrary number between 0 (good) and 5 (bad) isallocated as a summary of the panel assessment of hand-feel.

The invention is illustrated by the following non-limiting examples.

EXAMPLES Silica Example 1

A silica gel was prepared by mixing 40 percent by weight sulphuric acidwith sodium silicate (molar ratio 3.3:1; SiO₂: Na₂O), containing 25percent by weight SiO₂, in a reactant ratio corresponding to an excessof acid (0.25 Normal) and an SiO₂ concentration of 18.5 percent byweight The silica hydrosol produced was allowed to set to a hydrogel.After washing, the hydrogel was aged at pH 7.5 for 6 hours and the gelpH was subsequently adjusted down to 3.0. It was then flash dried.

The product had the properties shown in Table 1 below.

Silica Example 2

Neutral sodium silicate liquor (3.3:1; SiO₂: Na₂O ratio, 25 weightpercent SiO₂) was fed at a rate of 8.5 dm³ per minute and mixed withdilute sulphuric acid (40 percent by weight) being fed at a rate of 3.4dm³ per minute to produce a hydrosol with the composition 18.2 percentSiO₂, 12.3 percent sodium sulphate and 3 percent excess sulphuric add byweight. This hydrosol was allowed to set to a hydrogel on a continuouslymoving belt, as described in Example 1 of EP 0 244 168A with a beltresidence time of 10 minutes. The hydrogel was cut into lumps of about 1to 2 cm diameter and fed to a high shear mixer and disintegrated asdescribed in Example 1 of EP 0 244 168A with a residence time in thedisintegrator of 30 seconds. The hydrogel slurry produced was thenwashed in a series of three cyclones by a counter current arrangementusing fresh wash water. The slurry was pumped to each hydrocyclone at arate of 70 kg/min and a pressure of 0.34 Mpa, where it was split into anunderflow stream (20 kg/min and 50 wt. % solids (as hydrogel)) and anoverflow stream (50 kg/min and 2 wt. % solids (as hydrogel)). Wash water(mains water supply with 90 ppm soda as Na₂O), at a rate of 50 kg/min,was added to the final stage of the train. The hydrocyclones had 14 mmdiameter vortex finders and 9 mm spigots. After washing, the pH of thehydrogel slurry was adjusted to pH 6 and it was aged at pH 6 for 2hours. Subsequently the slurry pH was adjusted to pH 2.5 and the silicawas flash dried.

The properties of the product are given in Table 1 below. TABLE 1 PoreVolume Surface Area Example (ml/g) (m²/g) pH 1 2.0 300 3.0 2 1.9 500 2.5

Example 3

Preparation of Matting Agents.

The silicas of Examples 1 or 2 were mixed with a wax as indicated belowso that the resultant silica/wax mixture contained 9 weight percent ofthe wax or blend of waxes. This mixture of silica and wax was then fedto an AFG600 fluid bed mill (supplied by Alpine AG, Augsburg, Germany)the classifier speed and feed rate being set so as to produce amicronised product with a weight mean particle size in the range 5.0 to6.5 μm.

The waxes used are listed in Table 2 below. TABLE 2 Code Commercial NameType of wax x Wax Blend (as 80 wt. pts. hard microcrystalline wax,described in Crown ® 700¹ EP 0 541 359) 10 wt. pts. polyethylene wax,Polywax ® 2000¹ 10 wt. pts. plasticising microcrysytalline wax,Victory ®¹ y Crodawax WN 1442 Hydrophilic Polyethylene z Lanco ™ wax PEWHydrophilic Polyethylene 1555¹Available from Baker Petrolite

Example 4

Wax coated silicas, prepared as described in Example 3 were tested inlacquer 15 formulations as given below.

Alkyd Urethane Lacquer Part A Weight (g) Duramac 307-1297 alkyd/urethaneresin² 70.0 Xylene 18.0 n-Butyl Acetate 18.0 Iso Propyl Alcohol 18.0 Byk358 anti-foam³ 1.0 Disperbyk 103 wetting agent³ 1.5²Available from Eastman³Available from Byk Chemie GmbH

The above ingredients were thoroughly mixed by hand. Then, 100 g of PartA were mixed with an amount of the test silica appropriate to the testto be performed, as follows:

-   For Matting Efficiency, 4.00 g.-   For Optical Properties, 6.00 g.-   For Sedimentation Rate, 5.00 g.-   For Wet-in, 5.00 g.

For the Wet-In test the silica and Part A were mixed as describedhereinbefore in the Wet-in test method. For the other tests, Part A andthe silica were mixed using a Heidolph stirrer equipped with 4 cm Cowleshead for 10 minutes at 3000 rpm to form Part B. The sedimentation testwas performed using Part B. For the remaining tests, 100 g of Part Bwere then mixed with 20 g Desmodur L75 isocyanate cross-linker(available from Bayer GmbH) using a Heidolph stirrer equipped with a 4cm Cowles head for 1 minute at 1000 rpm. The mixture was left to standfor 30 minutes before tests for Matting Efficiency and OpticalProperties were carried out.

Acrylic Lacquer Weight (g) Paraloid B-66 acrylic resin⁴ 227.5 Toluene61.25 Xylene 61.25⁴Available from Rohm & Haas.

The above ingredients were mixed at 500-1000 rpm using a Heldolphstirrer equipped with a 4 cm Cowles head for 5 minutes and then sievedthrough a 45 μm sieve. 8.6 g silica were added and dispersed at 3000 rpmfor 10 minutes using a Heidolph stirrer equipped with a 4 cm Cowleshead. The resulting lacquer was used for the Gloss-up test.

The results of tests on the above formulations are summarised in Table4, below. TABLE 4 Wet-in⁶ Gloss- D50 Matting Haze Sediment (Hegman upSilica wax (μm) (60°) (%) Level (%) μm) rate Ex. 2 z 5.0 25 60 72 23 C4.4 Ex. 2 z 5.5 23 64 84 24 C 5.0 Ex. 1 z 5.5 20 58 78 26 C 5.3 Ex. 2 y5.0 19 64 90 23 SD 3.0 Ex. 1 y 5.5 18 65 74 24 VSD 3.5 Ex. 1 x 6.5 63 7062 35 C 4.2 Syloid — — 28 70 64 >100 5.5 ED 30⁵ Acematt — — 66 56 52 22C 7.9 OK412⁵⁵Commercially available matting agents.⁶C = Clear - no visible coarse particles greater than quoted Hegmanvalue; VSD = very slightly dirty - 1 to 6 coarse particles greater thanquoted Hegman value; SD = slightly dirty - 6 to 12 coarse particlesgreater than quoted Hegman value.

1. A silica matting agent comprising particulate amorphous silica in which the particles of silica have been treated with a hydrophilic polyolefin wax.
 2. A silica matting agent according to claim 1 in which the hydrophilic polyolefin wax has a molecular weight in the range 800 to
 30000. 3. A silica matting agent according to claim 1 in which the hydrophilic polyolefin wax has a melting point in the range 100 to 130° C.
 4. A silica matting agent according to claim 1 in which the hydrophilic polyolefin wax is present in an amount in the range 3 to 20 percent by weight with respect to untreated silica.
 5. A silica matting agent according to claim 1 in which the silica has a weight mean particle size in the range 2 to 20 μm.
 6. A silica matting agent according to claim 1 in which the untreated silica has a BET surface area in the range 100 to 800 m²g⁻¹.
 7. A silica matting agent according to claim 1 in which the untreated silica has a pore volume to nitrogen in the range 1.0 to 2.1 cm³g⁻¹.
 8. A silica matting agent according to claim 1 in which the untreated silica has an oil absorption in the range 100 to 500 cm³/100 g.
 9. A method of preparing a silica matting agent comprising passing particulate amorphous silica and a wax through a size reduction apparatus in which the wax is a hydrophilic polyolefin wax.
 10. A method of preparing a silica matting agent according to claim 9 in which the hydrophilic polyolefin wax has a weight mean particle size below 15 μm.
 11. A method of preparing a silica matting agent according to claim 9 in which the silica and wax reach a temperature in the range 100 to 150° C. as they pass through the size reduction apparatus. 